1. Field of the Invention
The present invention relates to an optical measurement system and a method thereof. More particularly, the present invention relates to a two-dimensional nanometric-scale displacement measurement system and a method thereof.
2. Description of Related Art
Generally, in a displacement measurement system with a grating, a coherent light beam is emitted into the grating, so that a plurality of diffraction beams are generated. Parts of the diffraction beams interfere with a reference light beam, so that periodic interference fringes are formed. Accordingly, when the grating moves, the displacement of the grating is calculated from variations of the above-described interference fringes. However, when the related technology was initially developed, the above-described displacement measurement system is simply used to measure the displacement in one-dimension due to the then fabricating technique of the grating.
With development of related technologies, multi-dimensional displacement measurement systems are provided. For example, in a displacement measurement system described in U.S. Pat. No. 5,666,196, gratings at a predetermined period respectively serve as alignment marks for the corresponding displacements in each dimension, so that the actual displacements are measured and recorded. Further, in a displacement measurement system described in U.S. Pat. No. 5,493,397, the actual displacements in each dimension are obtained by detecting amplitude changes of the interference fringes in each dimension. In the above-described multi-dimensional displacement measurement system, a plurality of photo sensors disposed in each dimension are used to read the amplitudes of the interference fringes for obtaining the actual displacements in each dimension, which in practice gives rise to an increase in costs and complexity of the displacement measurement system. Furthermore, the displacement measurement system having the photo sensors is difficult to be applied in practice.